Hi, I measured capacitance on some capacitors using my UNI-T 139S True RMS Multimeter without discharging them first. To be clear, I don't even know if they were charged because they are mounted on the PCB in USB charger that I disassembled (I disconnected it from the socket and after 1 hour I began disassebling it). Later I read in multimeter instructions that you should discharge capacitors first. In instructions that I got with it, in the Capacitance measurement section it says that the multimeter has overload protection: 600V-PTC.
I am wondering what damage it can cause to a multimeter ? Capacitor values that I measured are below.
USB Charger:
90 -260 VAC 50-60Hz
Capacitor values:
470μF 10V,
4.7μF 400V
UNI-T 139S safety standards: UT139S
Best Answer
Most multimeters test capacitance in one of two ways:
Both of these require small signals and small voltages. Applying large voltages to this can damage components. All meters are different, so consult the documentation of yours for any relevant information. Most manufacturers are void of operating details. But violating the limitations of the meter can indeed result in temporary or permanent damage. Typically a traditional glass or ceramic fuse will fail, preventing all operation. Usually these are accessible by removing the battery door, or on some models, the outer enclosure.
Measuring capacitance is more complicated than other modes because reference components may need to be switched in/out, or the meter may need to sample and electrically "kick" the capacitor, etc.
Some dedicated capacitor testers simply use diodes back-to-back across the terminals for protection, which effectively limit the usable signal to +/-0.6v and anything over this is shorted. I've heard of at least one other meter which keeps relay contacts closed across the terminals, until the "test" button is pressed. And both of these were damaged by connecting charged caps - a diode was welded shorted on the first, and relay contact welded closed on the second.
Your particular meter apparently uses at least a 600v positive-temperature-coefficient reset-able fuse, likely with other components such as clamping diodes, to reduce the chances of high voltages damaging it while in capacitance mode. This "polyfuse" is designed to "open" as it gets warm, disallowing further current to flow. After it cools it should work again. This is more convenient than a traditional fuse in that it doesn't need replacing and can be reused several times. However heating is slow, and if heated too quickly or too severely (by say, a large capacitor charged to a high voltage) it may never return to a functional status. Traditional fuses can be much faster-acting so would be preferred for this role. So in that sense, a polyfuse would not be my first choice for a capacitance testing mode, because it may let in too much current before it warms enough to open.
So to answer "how can the meter be damaged if the capacitor is not discharged", the capacitor itself could be damaged due to high currents, a fuse could be damaged if so equipped, protection diodes could permanently short, reference inductors could be vaporized, and/or other parts of the meter including D2A and A2D converters, even the CPU itself, rendering repair impossible.
Make it a habit - discharge all capacitors before testing (even little ones.)